Negative active material for rechargeable lithium battery and rechargeable lithium battery including same

ABSTRACT

A negative active material for a rechargeable lithium battery includes a composite of silicon and crystalline carbon, wherein the silicon has an average particle diameter (D 50 ) of about 10 nm to about 150 nm, and the crystalline carbon has an average particle diameter (D 50 ) of about 5 μm to about 20 μm, and an aspect ratio of about 4 to about 10.

CROSS-REFERENCE TO RELATED APPLICATION

Korean Patent Application No. 10-2018-0057197, filed on May 18, 2018, inthe Korean Intellectual Property Office, and entitled: “Negative ActiveMaterial for Rechargeable Lithium Battery and Rechargeable LithiumBattery Including Same,” is incorporated by reference herein in itsentirety.

BACKGROUND 1. Field

Embodiments are directed to a negative active material for arechargeable lithium battery and a rechargeable lithium batteryincluding the same.

2. Description of the Related Art

Technology development for realizing high capacity of a rechargeablelithium battery has been continuously made due to an increasing demandon a mobile equipment or a portable battery.

As an electrolyte of a rechargeable lithium battery, a lithium saltdissolved in an organic solvent has been used.

As for a positive active material of a rechargeable lithium battery, alithium-transition metal oxide having a structure capable ofintercalating lithium ions such as LiCoO₂, LiMn₂O₄, LiNi _(1−x)Co_(x)O₂(0<×<1), and the like has been used.

As for a negative active material, various carbon materials capable ofintercalating/deintercalating lithium such as artificial graphite,natural graphite and hard carbon or a Si-based active material such asSi, Sn, and the like may be used. In recent years, as high capacitybatteries, particularly high capacity per unit volume, are required, ahigh specific capacity of the negative electrode is required.Accordingly, research has been conducted on using a composite of siliconand carbon for the negative electrode. However, the composite of siliconand carbon has a problem that the volume expansion occurs remarkablyduring charging and discharging.

SUMMARY

Embodiments are directed to a negative active material for arechargeable lithium battery, the negative active material comprising acomposite of silicon and crystalline carbon, wherein the silicon has anaverage particle diameter (D50) of about 10 nm to about 150 nm, and thecrystalline carbon has an average particle diameter (D50) of about 5 μmto about 20 μm, and an aspect ratio of about 4 to about 10.

The negative active material may have an aspect ratio of about 1 toabout 2.5.

The silicon has an average particle diameter (D50) of about 40 nm toabout 120 nm.

The crystalline carbon may have an average particle diameter (D50) ofabout 5 μm to about 10 μm.

A mixing ratio of the silicon and the crystalline carbon may be a weightratio of about 1:9 to about 9:1.

The silicon-carbon composite may further include an amorphous carboncoating layer on the surface thereof.

An amount of the amorphous carbon coating layer may be about 10 wt % toabout 60 wt % based on total 100 wt % of the negative active material.

Embodiments are also directed to a rechargeable lithium batteryincluding a negative electrode including the negative active materialdescribed above, a positive electrode including a positive activematerial, and a non-aqueous electrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

Features will become apparent to those of skill in the art by describingin detail exemplary embodiments with reference to the attached drawingsin which:

FIG. 1 illustrates a schematic view showing a structure of a negativeactive material according to an embodiment.

FIG. 2 illustrates a schematic view showing a structure of arechargeable lithium battery according to an embodiment.

FIGS. 3A and 3B illustrate a CP-SEM (Controlled Pressure ScanningElectron Microscope) images of the negative active materialsmanufactured according to Example 1(a) and Reference Example 1(b).

FIGS. 4A and 4B illustrate views explaining an aspect ratio ofartificial graphite and a negative active material in the CP-SEM imagesof FIGS. 3A and 3B.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings; however, they may be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey exemplary implementations to those skilled in the art.

In the drawing figures, the dimensions of layers and regions may beexaggerated for clarity of illustration. It will also be understood thatwhen a layer or element is referred to as being “on” another layer orsubstrate, it can be directly on the other layer or substrate, orintervening layers may also be present. Like reference numerals refer tolike elements throughout.

A negative active material for a rechargeable lithium battery accordingto an embodiment includes a composite of silicon and crystalline carbon,wherein the silicon has an average particle diameter (D50) of about 10nm to about 150 nm and the crystalline carbon has an average particlediameter (D50) of about 5 μm to about 20 μm, and an aspect ratio ofabout 4 to about 10.

As used herein, when a definition is not otherwise provided, an averageparticle diameter (D50) indicates a particle diameter where a cumulativevolume in a particle distribution is about 50 volume %.

The silicon may have an average particle diameter (D50) of about 10 nmto about 150 nm. In some implementations, the silicon may have anaverage particle diameter (D50) of about 40 nm to about 120 nm. When theaverage particle diameter (D50) of the silicon is within the ranges, adeterioration of cycle-life characteristics may be minimized or avoided.

The crystalline carbon may have an average particle diameter (D50) ofabout 5 μm to about 20 μm, or, for example, about 5 μm to about 10 μm.When the average particle diameter (D50) of the crystalline carbon withthe ranges, deterioration of cycle-life characteristics may be minimizedor avoided.

The crystalline carbon may have an aspect ratio of about 4 to about 10.When the crystalline carbon has an aspect ratio within the range, asevere expansion of a negative electrode including this active materialand thus an excessive volume increase thereof, that is, deterioratedswelling characteristics that could occur during the charge anddischarge of a rechargeable lithium battery including the negativeelectrode may be minimized or avoided.

The aspect ratio may be determined from photographic images taken withCP-SEM.

In the silicon-carbon composite of the negative active materialaccording to an embodiment, the average particle diameter (D50) ofsilicon, the average particle diameter (D50) of crystalline carbon, andthe aspect ratio of the crystalline carbon are controlled within theabove-mentioned ranges. For example, the silicon-carbon composite mayinclude the silicon having an average particle diameter (D50) and thecrystalline carbon having a specific average particle diameter (D50) anda specific aspect ratio. In this case, Si may be more uniformlydispersed to improve cycle-life characteristics and swellingcharacteristics.

In a negative active material according to an embodiment, the negativeactive material composite may have an aspect ratio of about 1 to about2.5 In some implementations, the aspect ratio may be about 1 to about 2.When the silicon-carbon composite has an aspect ratio within the range,a negative active material may expand in a more uniform direction. Thus,the expansion of the negative active material in an electrode may beeffectively reduced.

In addition, when the silicon-carbon composite including silicon havingthe aforementioned specific average particle diameter (D50) andcrystalline carbon having a specific average particle diameter (D50) anda specific aspect ratio has an aspect ratio of about 1 to about 2.5,cycle-life characteristics and swelling characteristics may be furtherimproved.

The crystalline carbon may be natural graphite, artificial graphite, ora combination thereof.

In an embodiment, in the silicon-carbon composite, the silicon and thecrystalline carbon may be used in a weight ratio of about 1:9 to about9:1. In some implementations, the weight ratio may be about 5:5 to about8:2. When the silicon and the crystalline carbon are mixed within theweight ratio range, more excellent capacity and higher capacity thanthat of a general crystalline carbon negative active material may berealized. In addition, when the weight ratio is about 5:5 to about 8:2,a much higher capacity may be obtained.

The silicon-carbon composite may further include an amorphous carboncoating layer on the surface. The amorphous carbon may be petroleumpitch, coal pitch, or a combination thereof. The amorphous carboncoating layer may have a thickness of about 5 nm to about 1,000 nm. Thenegative active material includes about 10 wt % to about 60 wt % of anamorphous carbon layer based on 100 wt % of a total weight of thenegative active material.

When an amorphous carbon coating layer is further formed on the surfaceof the silicon-carbon composite, conductivity of the silicon-carboncomposite may be further improved, and thus performance may be improved.In addition, a direct contact between silicon and an electrolyte may bereduced, and thus, a resistance increase due to a generation ofbyproducts may be effectively suppressed.

The silicon-carbon composite may include a pore. For example, as anillustration of a structure of the silicon-carbon composite, acrystalline carbon core internally including a pore, an amorphous carbonshell formed on the surface of the core, Si particles dispersed insidethe pore, and amorphous carbon present inside the pore may be provided.

The structure of the silicon-carbon composite is schematically shown inFIG. 1 as an example. A silicon-carbon composite 221 shown in FIG. 1includes crystalline carbon 223, silicon 225, and amorphous carbon 227,wherein the amorphous carbon 227 is present among assemblies of thecrystalline carbon 223 and the silicon 225 and in addition, surroundsthe surfaces of the assemblies.

In addition, the silicon-carbon composite may internally include a pore.The pore may be pipe-shaped or sheet-shaped and may form a mesh networkinside the core. In the silicon-carbon composite, porosity may beappropriately adjusted. For example, porosity may range from about 2volume % to about 50 volume % based on a total volume of thesilicon-carbon composite.

A negative active material according to the embodiment may be preparedby mixing silicon particles and artificial graphite and optionally,amorphous carbon in a solvent and then, spray-drying and heat-treatingthis mixed solution. The solvent may be isopropyl alcohol, ethanol,methanol, or a combination thereof. The spray drying process may beperformed at about 90° C. to about 120° C., and the heat treatmentprocess may be performed at about 900° C. to about 1,000° C. The heattreatment process may be performed under an N₂ atmosphere, an argonatmosphere, a H₂ atmosphere, or a combination thereof.

When the mixed solution drying process is performed by spray-drying,powder may be easily obtained through a mass production.

When the spray dry process is performed within the temperature range, asolvent may be easily volatilized and removed. When the heat treatmentprocess is performed within the temperature range, conductivity of anegative active material may be improved, and the strength thereof maybe maintained. When amorphous carbon is used, the amorphous carbon maybe carbonized and thus may increase the effect of improving conductivityof the negative active material and maintaining strength thereof.

The negative active material according to an embodiment may include thecomposite as a first active material and may include crystalline carbonas a second active material. The crystalline carbon may be artificialgraphite, natural graphite, or a combination thereof. When an activematerial is prepared through mixing in this way, high-capacity may beobtained without deteriorating performance. A mixing ratio of the firstactive material and the second active material may be appropriatelyadjusted, for example, in a weight ratio of about 10:90 to about 40:60.

According to an embodiment, a rechargeable lithium battery may include anegative electrode; a positive electrode; and an electrolyte.

The negative electrode may include a current collector and a negativeactive material layer formed on the current collector and including thenegative active material.

In the negative active material layer, the negative active material maybe included in an amount of about 95 wt % to about 99 wt % based on thetotal weight of the negative active material layer.

The negative active material layer may include a binder, and optionally,a conductive material. In the negative active material layer, an amountof the binder may be about 1 wt % to about 5 wt % based on a totalweight of the negative active material layer. When the negative activematerial layer further includes the conductive material, the negativeactive material layer may include about 90 wt % to about 98 wt % of thenegative active material, about 1 wt % to about 5 wt % of the binder,and about 1 wt % to about 5 wt % of the conductive material.

The binder may improve binding properties of negative active materialparticles with one another and with a current collector. The binder mayinclude a non-aqueous binder, an aqueous binder, or a combinationthereof.

The non-aqueous binder may include polyvinylchloride, carboxylatedpolyvinylchloride, polyvinylfluoride, an ethylene oxide-containingpolymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide,polyimide, or a combination thereof.

The aqueous binder may include a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber (SBR), an acrylonitrile-butadiene rubber, anacrylic rubber, a butyl rubber, a fluorine rubber, an ethylene propylenecopolymer, polyepichlorohydrine. polyphosphazene, polyacrylonitrile,polystyrene, an ethylene propylene diene copolymer, polyvinylpyridine,chlorosulfonated polyethylene, latex, a polyester resin, an acrylicresin, a phenolic resin, an epoxy resin, polyvinyl alcohol, or acombination thereof.

When the aqueous binder is used as the negative electrode binder, acellulose-based compound may be further used as a thickener to provideviscosity. The cellulose-based compound includes one or more ofcarboxymethyl cellulose, hydroxypropylmethyl cellulose, methylcellulose, or alkali metal salts thereof. The alkali metal may be Na, K,or Li. Such a thickener may be included in an amount of about 0.1 partsby weight to about 3 parts by weight based on 100 parts by weight of thenegative active material.

The conductive material may be included to provide electrodeconductivity. Any electrically conductive material that does not cause achemical change may be used as a conductive material. Examples of theconductive material include a carbon material such as natural graphite,artificial graphite, carbon black, acetylene black, ketjen black, acarbon fiber, or the like; a metal-based material of a metal powder or ametal fiber including copper, nickel, aluminum, silver, or the like; aconductive polymer such as a polyphenylene derivative, or a mixturethereof.

The current collector may include one selected from a copper foil, anickel foil, a stainless steel foil, a titanium foil, a nickel foam, acopper foam, a polymer substrate coated with a conductive metal, or acombination thereof.

The positive electrode may include a current collector and a positiveactive material layer formed on the current collector and including apositive active material.

The positive active material may be a compound (lithiated intercalationcompound) that is capable of intercalating and deintercalating lithium.For example, the positive active material may be one or more compositeoxides of a metal selected from cobalt, manganese, nickel, and acombination thereof, and lithium. For example, the positive activematerial may have a compound represented by one of the followingchemical formulae. Li_(a)A_(1−b)X_(b)D₂ (0.90≤a≤1.8, 0≤b≤0.5);Li_(a)A_(1−b)X_(b)O_(2−c)D_(c) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)E_(1−b)X_(b)O_(2−c)D_(c) (90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)E_(2−b)X_(b)O_(4−c)D_(c) (90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05);Li_(a)Ni_(1−b−c)Co_(b)X_(c)D_(α)(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2);Li_(a)Ni_(1−b−c)Co_(b)X_(c)O_(2−α)T_(α) (0.90≤a≤1.8, 0≤b≤0.5, 0≤c ≤0.05,0<α<2); Li_(a)Ni_(1−b−c)Co_(b)X_(c)O_(2−α)T₂ (0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, 0<α<2); Li_(a)Ni_(1−b−c)Mn_(b)X_(c)D_(α) (0.90≤a≤1.8, 0≤b≤0.5,0≤c≤0.05, 0<α≤2); Li_(a)Ni_(1−b−c)Mn_(b)X_(c)O_(2−α)T_(α) (0.90≤a≤1.8,0≤b≤0.5, 0≤c≤0.05, 0<α<2);Li_(a)Ni_(1−b−c)Mn_(b)X_(c)O_(2−α)T₂(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05,0<α<2); Li_(a)Ni_(b)E_(c)G_(d)O₂ (0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5,0.001≤d≤0.1); Li_(a)Ni_(b)Co_(c)Mn_(d)G_(e)O₂ (0.90≤a≤1.8, 0≤b≤0.9,0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li_(a)NiG_(b)O₂ (0.90≤a≤1.8,0.001≤b≤0.1) Li_(a)CoG_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1);Li_(a)Mn_(1−b)G_(b)O₂ (0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn₂G_(b)O₄(0.90≤a≤1.8, 0.001≤b≤0.1); Li_(a)Mn_(1−g)G_(g)PO₄ (0.90≤a≤1.8, 0≤g≤0.5);QO₂; QS₂; LiQS₂; V₂O₅; LiV₂O₅; LiZO₂; LiNiVO₄; Li₍₃₋₁₎J₂(PO₄)₃(0≤f≤2);Li⁽³⁻¹⁾Fe₂(PO₄)₃ (0≤f≤2); Li_(a)FePO₄ (0.90≤a≤1.8).

In the chemical formulae, A is selected from Ni, Co, Mn, and acombination thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr,V, a rare earth element, and a combination thereof; D is selected fromO, F, S, P, and a combination thereof; E is selected from Co, Mn, and acombination thereof; T is selected from F, S, P, and a combinationthereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and acombination thereof; Q is selected from Ti, Mo, Mn, and a combinationthereof; Z is selected from Cr, V, Fe, Sc, Y, and a combination thereof;and J is selected from V, Cr, Mn, Co, Ni, Cu, and a combination thereof.

The compounds may have a coating layer on the surface or may be mixedwith another compound having a coating layer. The coating layer mayinclude at least one coating element compound selected from an oxide ofa coating element, a hydroxide of a coating element, an oxyhydroxide ofa coating element, an oxycarbonate of a coating element, and a hydroxylcarbonate of a coating element. The compound for the coating layer maybe amorphous or crystalline. The coating element included in the coatinglayer may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As,Zr, or a mixture thereof. By using these elements in the compound, thecoating layer may be disposed in a method having no adverse influence onproperties of a positive active material. For example, the method mayinclude a suitable coating method such as spray coating, dipping, or thelike.

In the positive electrode, the amount of the positive active materialmay be about 90 wt % to about 98 wt % based on the total weight of thepositive active material layer.

The positive active material layer may further include a binder and aconductive material. The amount of each of the binder and the conductivematerial may be about 1 wt % to about 5 wt % based on the total weightof the positive active material layer.

The binder may serve to adhere the positive active material particleswith one another and to adhere the positive active material to a currentcollector. Examples of the binder include polyvinyl alcohol,carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose,polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, anethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane,polytetrafluoroethylene, polyvinylidene fluoride, polyethylene,polypropylene, a styrene-butadiene rubber, an acrylatedstyrene-butadiene rubber, an epoxy resin, nylon, or the like.

The conductive material may be included to provide electrodeconductivity. A suitable electrically conductive material that does notcause a chemical change may be used as a conductive material. Examplesof the conductive material may include a carbon material such as naturalgraphite, artificial graphite, carbon black, acetylene black, ketjenblack, a carbon fiber, or the like; a metal-based material of a metalpowder or a metal fiber including copper, nickel, aluminum, silver, orthe like; a conductive polymer such as a polyphenylene derivative, orthe like, or a mixture thereof.

The current collector may include, for example, an aluminum foil, anickel foil, or a combination thereof.

The electrolyte may include a non-aqueous organic solvent and a lithiumsalt.

The non-aqueous organic solvent may serve as a medium for transmittingions taking part in the electrochemical reaction of a battery.

The non-aqueous organic solvent may include a carbonate-based,ester-based, ether-based, ketone-based, alcohol-based, or aproticsolvent.

The carbonate based solvent may include dimethyl carbonate (DMC),diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropylcarbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate(MEC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), or the like. The ester-based solvent may include methylacetate, ethyl acetate, n-propyl acetate, dimethylacetate,methylpropionate, ethylpropionate, decanolide, mevalonolactone,caprolactone, or the like. The ether-based solvent may include dibutylether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran,tetrahydrofuran, or the like. The ketone-based solvent includescyclohexanone or the like. The alcohol-based solvent include ethylalcohol, isopropyl alcohol, or the like. Examples of the aprotic solventmay include a nitrile such as R-CN (where R is a C2 to C20 linear,branched, or cyclic hydrocarbon group, or may include a double bond, anaromatic ring, or an ether bond), an amide such as dimethylformamide,dioxolanes such as 1,3-dioxolane, a sulfolane, or the like.

The organic solvent may be used alone or in a mixture. When the organicsolvent is used in a mixture, the mixture ratio may be controlled inaccordance with a desirable battery performance.

When the non-aqueous organic solvent is used in a mixture, the mixturemay be a mixed solvent of a cyclic carbonate and a linear (chain)carbonate, a mixed solvent of a cyclic carbonate and a propionate basedsolvent, or a mixed solvent of a cyclic carbonate, linear carbonate, anda propionate based solvent. The propionate based solvent may bemethylpropionate, ethylpropionate, propylpropionate, or a combinationthereof.

When the cyclic carbonate and the linear carbonate or the cycliccarbonate and the propionate based solvent are mixed, they may be mixedin a volume ratio of about 1:1 to about 1:9. Thus, performance of anelectrolyte solution may be improved. IWhen the cyclic carbonate, thelinear carbonate, and the propionate based solvent are mixed, they maybe mixed in a volume ratio of about 1:1:1 to about 3:3:4. The mixingratios of the solvents may be appropriately adjusted according todesirable properties.

The organic solvent may further include an aromatic hydrocarbon-basedorganic solvent in addition to the carbonate-based solvent. Thecarbonate-based solvent and the aromatic hydrocarbon-based organicsolvent may be mixed in a volume ratio of about 1:1 to about 30:1.

The aromatic hydrocarbon-based organic solvent may be an aromatichydrocarbon-based compound of Chemical Formula 1.

In Chemical Formula 1, R₁ to R6 may be the same or different and may beselected from hydrogen, a halogen, a C10 to C10 alkyl group, a haloalkylgroup, or a combination thereof.

Examples of the aromatic hydrocarbon-based organic solvent may includebenzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene,1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene,chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene,1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene,iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene,1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene,2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene,2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene,2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene,2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene,2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene,2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, or a combinationthereof.

The electrolyte may further include an additive for improving thecycle-life of a battery. The additive may be or include vinylenecarbonate or an ethylene carbonate-based compound of Chemical Formula 2.

In Chemical Formula 2, R₇ and R₈ may be the same or different and may beselected from hydrogen, a halogen, a cyano group (CN), a nitro group(NO₂), or a fluorinated C1 to C5 alkyl group, provided that at least oneof R₇ and R₈ is selected from a halogen, a cyano group (CN), a nitrogroup (NO₂), and a fluorinated C1 to C5 alkyl group, and R₇ and R₈ arenot simultaneously hydrogen.

Examples of the ethylene carbonate-based compound may be difluoroethylenecarbonate, chloroethylene carbonate, dichloroethylene carbonate,bromoethylene carbonate, dibromoethylene carbonate, nitroethylenecarbonate, cyanoethylene carbonate. or fluoroethylene carbonate. Theamount of the additive for improving cycle-life may be used within asuitable range.

The lithium salt dissolved in an organic solvent supplies a battery withlithium ions, basically operates the rechargeable lithium battery, andimproves transportation of the lithium ions between positive andnegative electrodes. Examples of the lithium salt may include at leastone supporting salt selected from LiPF₆, LiBF₄, LiSbF₆, LiAsF₆,LiN(SO₂C₂F₅)₂, Li(CF₃SO₂)₂N, LiN (SO₃C₂F₅)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂,LiAlCl₄, LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂), wherein, x and y arenatural numbers, for example, an integer ranging from 1 to 20), LiCl,LiI, and LiB(C₂O₄)₂ (lithium bis(oxalato) borate: LiBOB). Aconcentration of the lithium salt may range from about 0.1 M to about2.0 M. When the lithium salt is included at the above concentrationrange, an electrolyte may have excellent performance and lithium ionmobility due to optimal electrolyte conductivity and viscosity.

The rechargeable lithium battery may further include a separator betweenthe negative electrode and the positive electrode, depending on a typeof the battery. Examples of a suitable separator material includepolyethylene, polypropylene, polyvinylidene fluoride, and multi-layersthereof having two or more layers. The separator may be a mixedmultilayer such as a polyethylene/polypropylene double-layeredseparator, a polyethylene/polypropylene/polyethylene triple-layeredseparator, or a polypropylene/polyethylene/polypropylene triple-layeredseparator.

FIG. 2 illustrates an exploded, cutaway perspective view of arechargeable lithium battery according to an embodiment (not showing acap plate, electrode tabs, etc. of a complete battery). The rechargeablelithium battery according to an embodiment is illustrated as a prismaticbattery as an example. The rechargeable lithium battery may be one ofvariously-shaped batteries such as a cylindrical battery, a pouchbattery, or the like.

Referring to FIG. 2, a rechargeable lithium battery 100 according to anembodiment may include an electrode assembly 40 manufactured by windinga separator 30 disposed between a positive electrode 10 and a negativeelectrode 20, and a case 50 housing the electrode assembly 40. Anelectrolyte may be impregnated in the positive electrode 10, thenegative electrode 20, and the separator 30.

The following Examples and Comparative Examples are provided in order tohighlight characteristics of one or more embodiments, but it will beunderstood that the Examples and Comparative Examples are not to beconstrued as limiting the scope of the embodiments, nor are theComparative Examples to be construed as being outside the scope of theembodiments. Further, it will be understood that the embodiments are notlimited to the particular details described in the Examples andComparative Examples.

Examples 1 to 6, Comparative Examples 1 to 5, and Reference Example 1

Artificial graphite having an average particle diameter (D50) and anaspect ratio shown in Table 1, silicon particles having an averageparticle diameter (D50) shown in Table 1, and petroleum pitch amorphouscarbon in a weight ratio of 40:40:20 were mixed in an isopropyl alcoholsolvent and dispersed by using a homogenizer to prepare a dispersion.

The dispersion was spray-dried at 120° C. by using a spray dryer.

The resulting spray-dried product was heat-treated at 1,000° C. in afurnace under an N₂ atmosphere to form a core of artificial graphite andsilicon particles and an amorphous carbon layer on the surface of thecore.

The obtained product was pulverized and sieved using a 400 mesh toproduce a first negative active material including a silicon-carboncomposite core of artificial graphite and silicon particles and anamorphous carbon layer formed on the surface of the core. This firstnegative active material was a negative active material including anartificial graphite core including pores thereinside, an amorphouscarbon shell formed on the surface of the core, Si particles inside thepores, and an amorphous carbon structure inside the pores. In theprepared first negative active material, a weight ratio of siliconparticles and artificial graphite was 5:5. In addition, in the preparedfirst negative active material, an amount of the amorphous carboncoating layer was 20 wt % based on 100 wt % of a total weight of thefirst negative active material, and a thickness of the amorphous carbonlayer was 30 nm.

15 wt % of the first negative active material and 85 wt % of naturalgraphite second negative active material were mixed to obtain a mixednegative active material.

94 wt % of the obtained mixed negative active material, 3 wt % of denkablack, and 3 wt % of a polyvinylidene fluoride binder were mixed in anN-methyl pyrrolidone solvent to prepare negative active material slurry.The slurry was coated onto a Cu foil current collector, dried andcompressed to manufacture a negative electrode.

A half-cell having a 1 C capacity of 3,600 mAh was manufactured usingthe negative electrode, a lithium metal counter electrode, and anelectrolyte. The electrolyte was prepared by dissolving 1.0 M LiPF₆ inethylene carbonate and diethyl carbonate (volume ratio of 50:50).

* Evaluation of Aspect Ratio of Negative Active Material

CP-SEM photographic images of the negative active materials of Example 1and Reference Example 1 were obtained. The images are respectively shownas FIGS. 3A and 3B. In FIGS. 3A and 3B, Si indicates silicon, and Grindicates graphite. Referring to the CP-SEM images, an aspect ratio ofthe artificial graphite (refer to FIG. 4A was determined. In addition,an aspect ratio of the negative active material (refer to FIG. 4B) wasdetermined. The results are shown in Table 1. In FIGS. 3A and 3B, theregion indicated by the arrow with the symbol “Gr” is used to determinethe aspect ratio of the artificial graphite as shown in FIG. 4A, and theregion marked with the dotted line is used to determine the aspect ratioof the negative active material as shown in FIG. 4B.

* Evaluation of Cycle-Life Characteristics

Half-cells including negative electrodes including each negative activematerial manufactured according to Examples 1 to 6, Comparative Examples1 to 5, and Reference Example 1 were charged and discharged at 0.5 C for100 times. Capacity ratios (%) of the discharge capacity of the 100thcharge and discharge cycle relative to the discharge capacity of thefirst charge and discharge cycle are shown in Table 1 as cycle-lifevalues.

* Thickness Expansion Rate

Half-cells manufactured by using negative electrodes respectivelyincluding the negative active materials according to Examples 1 to 6,Comparative Examples I to 5, and Reference Example 1 were charged anddischarged 50 times at 0.5 C. The thickness of each of the battery cellswas respectively measured before the charge and discharge and after the50 times charges and discharges. Then, a thickness ratio based on thethickness after the 50 charges and discharges relative to the thicknessbefore the charge and discharge was calculated. The results are shown asan expansion rate in Table 1.

TABLE 1 Average particle Average particle diameter of Aspect ratioAspect ratio of Expansion diameter of Si artificial graphite ofartificial negative active Cycle-life rate (D50, nm) (D50) (μm) graphitematerial (%) (%) Comparative 100 5 1-3  1-2.5 73 31 Example 1Comparative 100 5 1-3  3-7   65 41 Example 2 Reference 100 5 4-10 3-7  66 43 Example 1 Comparative 100 20 4-10 3-7   63 44 Example 3Comparative 100 27 4-10 1-2.5 69 36 Example 4 Comparative 200 5 4-101-2.5 72 34 Example 5 Example 1 100 5 4-10 1-2.5 83 23 Example 2 100 94-10 1-2.5 81 26 Example 3 100 20 4-10 1-2.5 78 27 Example 4 50 5 4-101-2.5 86 19 Example 5 50 9 4-10 1-2.5 83 21 Example 6 50 20 4-10 1-2.581 24

As can be seen in Table 1 battery cells including negative activematerials including silicon having an average particle diameter (D50) of10 nm to 150 nm and artificial graphite having an average particlediameter (D50) of 5 μm to 20 μm and an aspect ratio of 4 to 10, and inaddition, having an aspect ratio of 1 to 2.5 with specific valuesaccording to Examples 1 to 6, exhibited excellent cycle-lifecharacteristics and a low expansion rate compared with battery cellsincluding negative active materials according to Comparative Examples 1to 6 and Reference Example 1, which were outside of the values.

Examples 7 to 14 and Reference Example 2

Artificial graphite having an average particle diameter (D50) and anaspect ratio shown in Table 2, silicon particles having an averageparticle diameter (D50) shown in Table 2, and petroleum pitch amorphouscarbon in a weight ratio were mixed in an isopropyl alcohol solvent anddispersed by using a homogenizer to prepare a dispersion. A weight ratioof the silicon particles and the artificial graphite was adjusted asshown in Table 2. In addition, an amount of the amorphous carbon wasadjusted, so that a coating layer might have an amount shown in Table 2in a final first negative active material.

The dispersion was spray-dried at 120° C. using a spray dryer.

The resulting spray-dried product was heat-treated at 1,000° C. in afurnace under an N₂ atmosphere to form a core of artificial graphite andsilicon particles and an amorphous carbon layer on the surface of thecore.

The obtained product was pulverized and sieved using a 400 mesh toproduce a first negative active material including a silicon-carboncomposite core of artificial graphite and silicon particles and anamorphous carbon layer formed on the surface of the core. This firstnegative active material was a negative active material including anartificial graphite core including pores thereinside, an amorphouscarbon shell formed on the surface of the core, Si particles inside thepores, and an amorphous carbon structure inside the pores. In theproduced first negative active material, weight ratios of siliconparticles and artificial graphite according to examples are shown inTable 2.The amount of the amorphous carbon coating layer in the producedfirst negative active material is shown in Table 2.

15 wt % of the first negative active material and 85 wt % of naturalgraphite as a second negative active material were mixed to obtain amixed negative active material.

94 wt % of the obtained mixed negative active material, 3 wt % of denkablack, and 3 wt % of a polyvinylidene fluoride binder were mixed in anN-methyl pyrrolidone solvent to prepare negative active material slurry.The slurry was coated onto a Cu foil current collector, dried andcompressed to manufacture a negative electrode.

A half-cell having a 1 C capacity of 3,600 mAh was manufactured usingthe negative electrode, a lithium metal counter-electrode, and anelectrolyte. The electrolyte was prepared by dissolving 1.0 M LiPF₆ inethylene carbonate and diethyl carbonate (volume ratio of 50:50).

* Evaluation of Cycle-life Characteristics

Half-cells including negative electrodes respectively including thenegative active material manufactured according to Examples 7 to 14 andReference Example 2 were charged and discharged at 0.5 C for 100 times.Capacity ratios (%) of the discharge capacity of the 100th charge anddischarge cycle relative to the discharge capacity of the first chargeand discharge cycle are shown in Table 2 as a cycle-life value.

* Thickness Expansion Rate

Half-cells including negative electrodes respectively including thenegative active materials according to Examples 7 to 14 and ReferenceExample 2 were charged and discharged 50 times at 0.5 C. The thicknessof the battery cells was respectively measured before and after the 50time charging and discharging. Then, the thickness ratio of thethickness after 50 times charging and discharging relative to thethickness before the charges and discharges was calculated, and theresults are shown as an expansion rate in Table 2.

TABLE 2 Amount of amorphous carbon Si: coating layer Average particleartificial (wt % based on a Average particle diameter of Aspect ratioAspect ratio of graphite total amount, 100 Expansion diameter of Siartificial graphite of artificial negative active (weight wt % ofnegative Cycle-life rate (D50) (nm) D50 (μm) graphite material ratio)active material) (%) (%) Example 7 100 5 4-10 1-2.5 1:9 10 86 19 Example8 100 5 4-10 1-2.5 9:1 10 76 38 Example 9 100 5 4-10 1-2.5 5:5 10 80 26Example 10 100 5 4-10 1-2.5 4:6 10 83 24 Example 11 100 5 4-10 1-2.5 1:960 88 16 Example 12 100 5 4-10 1-2.5 9:1 60 79 35 Example 13 100 5 4-101-2.5 5:5 60 86 18 Example 14 100 5 4-10 1-2.5 4:6 60 87 17 Reference100 5 4-10 1-2.5 5:5 5 70 37 Example 2

As can be seen in Table 2, the battery cells using the negative activematerial including silicon having an average particle diameter (D50) of10 nm to 150 nm and artificial graphite having an average particlediameter (D50) of 5 μm to 20 μm and an aspect ratio of 4 to 10 and inaddition, having an aspect ratio of 1 to 2.5 and including an amorphouscarbon coating layer in an amount of 10 wt % to 60 wt % according toExamples 7 to 14 exhibited excellent cycle-life characteristics comparedwith the battery cell including a negative active material including anamorphous carbon coating layer in too a small amount of 5 wt % accordingto Reference Example 2. For example, in case of Reference Example 2,which has the amount of the amorphous carbon coating layer of 5 wt %,Greatly deteriorated cycle-life characteristics were exhibited.

By way of summation and review, as an electrolyte of a rechargeablelithium battery, a lithium salt dissolved in an organic solvent has beenused.

As for a positive active material of a rechargeable lithium battery, alithium-transition metal oxide having a structure capable ofintercalating lithium ions such as LiCoO₂, LiMn₂O₄, LiNi_(1−x)Co_(x)O₂(0<×<1), and the like has been used.

As for a negative active material, various carbon materials capable ofintercalating/deintercalating lithium such as artificial graphite,natural graphite and hard carbon or a Si-based active material such asSi, Sn, or the like may be used. In recent years, as high capacitybatteries, particularly high capacity per unit volume, become desirable,a high specific capacity of the negative electrode is desired.Accordingly, research has been conducted on using a composite of siliconand carbon for the negative electrode. However, the composite of siliconand carbon may have disadvantages that a volume expansion occursremarkably during charging and discharging.

Embodiments may provide a negative electrode for a rechargeable lithiumbattery capable of suppressing expansion during charge and dischargeeffectively and exhibiting excellent cycle-life characteristics.

Embodiments provide a rechargeable lithium battery including thenegative active material.

Example embodiments have been disclosed herein, and although specificterms are employed, they are used and are to be interpreted in a genericand descriptive sense only and not for purpose of limitation. In someinstances, as would be apparent to one of ordinary skill in the art asof the filing of the present application, features, characteristics,and/or elements described in connection with a particular embodiment maybe used singly or in combination with features, characteristics, and/orelements described in connection with other embodiments unless otherwisespecifically indicated. Accordingly, it will be understood by those ofskill in the art that various changes in form and details may be madewithout departing from the spirit and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. A negative active material for a rechargeablelithium battery, the negative active material comprising a composite ofsilicon and crystalline carbon, wherein the silicon has an averageparticle diameter (D50) of about 10 nm to about 150 nm, and thecrystalline carbon has an average particle diameter (D50) of about 5 μmto about 20 μm, and an aspect ratio of about 4 to about
 10. 2. Thenegative active material as claimed in claim 1, wherein the negativeactive material has an aspect ratio of about 1 to about 2.5.
 3. Thenegative active material as claimed in claim 1, wherein the silicon hasan average particle diameter (D50) of about 40 nm to about 120 nm. 4.The negative active material as claimed in claim 1, wherein thecrystalline carbon has an average particle diameter (D50) of about 5 μmto about 10 μm.
 5. The negative active material as claimed in claim 1,wherein a mixing ratio of the silicon and the crystalline carbon is aweight ratio of about 1:9 to about 9:1.
 6. The negative active materialas claimed in claim 1, wherein the silicon-carbon composite furtherincludes an amorphous carbon coating layer on the surface thereof. 7.The negative active material as claimed in claim 1, wherein an amount ofthe amorphous carbon coating layer is about 10 wt % to about 60 wt %based on total 100 wt % of the negative active material.
 8. Arechargeable lithium battery, comprising a negative electrode includingthe negative active material as claimed in claim 1; a positive electrodeincluding a positive active material; and a non-aqueous electrolyte.